Piezoelectric crystals have been used for many years as frequency standards due to their stable resonant properties. Recognition that these crystals are sensitive to mechanical stress lead to the initial concept of a piezoelectric resonant transducer for the measurement of strain produced in the crystal by external forces. The frequency shift caused by mechanical strain is particularly convenient for obtaining measurements of applied force in digital form.
One prior art accelerometer uses two AT-cut thickness shear mode crystals preloaded in compression by a spring supported between halves of a split mass. When an acceleration is applied along its sensitive axis, the compressive strain is increased in one crystal and decreased in the other. This differential compressive strain shifts the resonant frequencies of the two crystals thereby providing a digital measure of the acceleration applied along the sensitive axis. This accelerometer requires a close matching of the thermal sensitivities of the crystals if good performance is to be attained. Also, accurate thermal tracking between the two crystals is necessary because the common mode rejection capabilities of the differential accelerometer will be limited by the temperature mismatch between the two ends. Therefore, the performance potential of two AT-cut crystals differentially measuring acceleration is low due to the residual temperature sensitivity of the resonant section and the design difficulties in attempting to match temperatures at both active resonant sections.